The ease with which diesel engines start diminishes at low temperatures. On the one hand, an increase in friction moment is responsible for this increasing reluctance to start. On the other hand, leakage and heat losses during compression of the air reduce the final compression pressure and the final compression temperature to such an extent that starting without an auxiliary starter is no longer possible. The temperature limit at which starting becomes impossible generally depends on the engine design. For example, direct injection diesel engines with a one-part combustion chamber (called DI engines below) have lower heat losses and, thus, exhibit better starting behavior, than precombustion chamber and swirl chamber engines which have a divided combustion chamber.
Sheathed-element glow plugs are ordinarily used in smaller-volume DI engines, as well as in engines with a divided combustion chamber, in order to improve starting behavior. The glow-plug of the element extends into the combustion chamber or chambers of the engine. In addition to sheathed-element glow plugs, glow plugs with an exposed incandescent wire are also common. In large-volume diesel engines auxiliary starting is often based on preheating of the air drawn in to the engine by means of electrically heatable heater plugs or heating flanges.
In order to be able to start the diesel engine at low temperatures, the heating device, (for example, the sheathed-element glow plugs), is supplied with electrical power from the vehicle battery. The heating device converts the electrical power to heat. The electrical power available during this pre-ignition heating process is limited by the maximum possible discharge current of the battery. The maximum possible discharge current diminishes with diminishing temperature and, therefore, behaves opposite to the power requirement as a function of temperature. A temperature sensor incorporated in an ignition control device controls the duration of the required pre-ignition time. The end of the pre-ignition time, (i.e., the onset of readiness to start), is noted, for example, by extinction of a control light.
Because of the limited feed power, the pre-ignition time in now common small-volume DI engines is about 2 to 5 seconds (during a cold start). This relatively long waiting time before the onset of the starting readiness condition represents a certain safety risk in hazardous situations in which immediate driving off is necessary, as well as an adverse effect on comfort.
Various proposals have already been made for further development of pre-ignition units. For example, it is known from DE 37 13 835 A1 that glow plugs of a diesel engine can be operated with an overvoltage in order to shorten the heat-up time. The operation of 6-volt glow plugs with a voltage of 12-volts is mentioned as an example of this approach.
EP 0 420 379 B1 proposes supplying glow plugs for a diesel engine from a series circuit of two capacitors that are connected in parallel during charging. This parallel/series alternating circuit feeds the glow plugs with twice the battery voltage.
DE 37 43 317 A1 describes the general state of the art concerning current supply in motor vehicles. It discloses the supply of other consumers (i.e., not glow plugs or the like) from a dc intermediate circuit with increased intermediate circuit voltage. A dc intermediate circuit with increased intermediate circuit voltage is also described, for example, in DE 195 32 163 A1.
Under practical conditions, the previous proposals to supply sheathed-element glow plugs and the like from capacitor accumulators have not gained acceptance. Other proposals, which supply glow plugs with a slight overvoltage obtained from the vehicle battery do shorten the required ignition times, but apparently have not gained widespread acceptance.